In recent years, there have been rapidly increasing demands for not only electricity storage systems for small-sized and high energy density applications, for example, information-related apparatus, communication apparatus, i.e., personal computers, video cameras, digital cameras, portable telephones, and smartphones; but also batteries with large capacity, high output and high energy density which can be used for electric vehicles, hybrid vehicles, and auxiliary power systems of fuel-cell vehicles. Moreover, there have been increasing demands for batteries which can be used for a long time even in electricity storage systems for large-sized and high power applications, for example, electric power storages. As one of the candidates for such electricity storage systems, nonaqueous electrolytic solution batteries have been under active development, such as lithium ion batteries, lithium batteries, and lithium ion capacitors.
Lithium secondary batteries mainly include a positive electrode, a nonaqueous electrolytic solution, and a negative electrode. As negative electrodes for lithium secondary batteries, known are, for example, metal lithium, metal compounds (for example, elemental metals, oxides, alloys with lithium, and the like) capable of occluding and releasing lithium, carbon materials, and the like. In particular, lithium secondary batteries where carbon materials capable of occluding and releasing lithium such as corks, artificial graphite, natural graphite, and the like are used have been put into wide practical use. For example, it is reported that in a lithium secondary battery where a highly crystallized carbon material such as natural graphite and artificial graphite is used as a negative electrode material, a nonaqueous solvent in a nonaqueous electrolytic solution may be reductively decomposed on the surface of a negative electrode upon charging, resulting in generation of decomposition products or gases. This may interfere with the desired electrochemical reactions of the battery, which in turn, may decrease cycle characteristics.
Further, in a lithium secondary battery where metal lithium or an alloy thereof, an elemental metal such as silicon and tin, or an oxide is used as a negative electrode material, pulverization of the negative electrode material is promoted during cycles although it has a high initial capacity. Therefore, a nonaqueous solvent is more susceptible to reductive decomposition as compared with a negative electrode made of a carbon material. As a result, the charge/discharge efficiency at the first cycle is known to be decreased due to an increased initial irreversible capacity of the battery. It is also known that this may significantly decrease battery performances such as battery capacity and cycle characteristics. A negative electrode may react with lithium cations or a solvent of an electrolytic solution when lithium cations are intercalated into the negative electrode upon charging at the first cycle. This may form a film containing lithium oxide, lithium carbonate, and lithium alkylcarbonate as the main components on the surface of the negative electrode. This film on the surface of the electrode which is called a Solid Electrolyte Interface (SEI) may, in nature, have significant impacts on battery performance. For example, it may reduce reductive decomposition of a solvent to prevent deterioration of battery performance. As described above, one of the disadvantages is that lithium may not be smoothly occluded into and released from a negative electrode due to adverse effects such as accumulation of decomposition products and generation of gases from a nonaqueous solvent, and pulverization of a negative electrode material, resulting in significant deterioration of battery characteristics such as cycle characteristics.
Meanwhile, as a positive electrode, known are, for example, LiCoO2, LiMn2O4, LiNiO2, LiFePO4, and the like. It is reported that in lithium secondary batteries where these materials are used, a nonaqueous solvent in a nonaqueous electrolytic solution may partly undergo local oxidative decomposition at the interface between a positive electrode material and the nonaqueous electrolytic solution when the temperature is increased during charging. This results in generation of decomposition products and gases. As a result, the desired electrochemical reaction of the battery may be interfered with, which in turn, may decrease battery performances such as cycle characteristics. As in the negative electrode, a film may also be formed on the surface of the positive electrode due to oxidatively decomposed products. This film is also known to play an important role. For example, oxidative decomposition of a solvent may be prevented, and the battery gas yield may be reduced.
As described above, conventional lithium secondary batteries have a problem in that decomposition products and gases generated when a nonaqueous electrolytic solution decomposes on a positive electrode and a negative electrode may interfere with the movement of lithium ions, and may cause the expansion of a battery. These may be responsible for decreased battery performance.
In order to overcome the above problems and further improve battery performance such as long term durability and output characteristics, it is important to form an SEI having a high ion conductivity, a low electron conductivity, and a long-term stability. To this end, attempts have been widely made for intentionally forming a good SEI by adding a small amount (usually 0.01 mass % or more and 10 mass % or less) of a compound called an additive to an electrolytic solution.
For example, in a secondary battery where a graphite-based negative electrode with a high degree of crystallinity is used, a nonaqueous electrolytic solution containing, for example, vinylene carbonate, vinylethylene carbonate, maleic anhydride, phthalic anhydride, and/or the like has been used to minimize decomposition of the nonaqueous electrolytic solution to obtain a high capacity. Further, attempts have been made for improving storage properties and cycle characteristics at high temperature (Patent Documents 1, 2, 3, and 4). However, these are still less than satisfactory. For example, use of a nonaqueous electrolytic solution including ethylene carbonate as the main solvent and containing 0.01 to 10.0 mass % of vinylene carbonate relative to ethylene carbonate can not sufficiently prevent an increased internal resistance of a battery when stored at high temperature. Moreover, Patent Document 5 discloses a nonaqueous electrolyte battery in which an electrolytic solution containing a diisocyanate compound is used. This represents an attempt for improving the long-term storage reliability of a battery and others.
Meanwhile, the following methods have been considered: a method of improving the thermal stability of an electrolytic solution by using lithium bis(oxalato)borate as an Li salt in place of a common Li salt such as LiPF6 and LiBF4; and a method of improving life-time performance by preventing generation of hydrofluoric acid which is responsible for elution of transition metal contained in a positive-electrode active material (Patent Document 6). Further, a nonaqueous electrolytic solution is disclosed which contains a lithium salt having an oxalato complex such as lithium bis(oxalato)borate as an anion and at least one film-forming agent selected from the group consisting of vinylene carbonate, vinylethylene carbonate, ethylene sulfite, and fluoroethylene carbonate (Patent Document 7).
A nonaqueous electrolytic solution is disclosed containing a phosphorus-boron complex and the like as an additive for forming an effective SEI, such as a lithium difluoro(oxalato)borate (Patent Document 8). Further, disclosed is a means for providing a lithium-ion secondary battery capable of having an outstanding regeneration output, which is configured to have a hard-carbon negative electrode and a predetermined capacity ratio of positive electrode/negative electrode, and contains a similar phosphorus-boron complex as a nonaqueous electrolytic solution (Patent Document 9). Further, proposals for improving input and output characteristics at low temperature are disclosed in which vinylene carbonate or fluoroethylene carbonate and lithium difluoro(bisoxalato)phosphate are included each in predetermined amounts (Patent Documents 10 and 11). Moreover, a proposal for improving battery characteristics such as battery capacity, cycle characteristics, and storage properties is disclosed in which an electrolytic solution is used containing lithium difluoro(oxalato-O,O′)borate or lithium tetrafluoro(oxalato-O,O′)phosphate and a carbonate ester of an unsaturated compound such as vinylene carbonate and vinylethylene carbonate as an additive (Patent Document 12).
Patent Document 13 describes a proposal for improving durability and loading characteristics such as cycles and storage, including at least one selected from the group consisting of compounds in which a triple bond is bonded to a ring structure via a single bond without via another functional group or a hetero atom (4-ethynylethylene carbonate, 4-ethynyl-1,3,2-dioxathiolane-2,2-dioxide, and the like); and further compounds such as LiPO2F2 and LiSO3F; lithium salts of oxalato complexes such as lithium bis(oxalato)borate, lithium difluorooxalatoborate, lithium tetrafluorooxalatophosphate, lithium difluorobis(oxalato)phosphate, and lithium tris(oxalato)phosphate; and carbonates having at least one carbon-carbon unsaturated bond or fluorine atom.
Patent Document 14 describes a proposal for improving discharge capacity, initial charge/discharge efficiency, and loading characteristics, including a lithium-transition metal composite oxide having the stratified rock-salt structure in a positive electrode, a nonaqueous solvent containing a fluorinated solvent (fluorinated carbonate and the like) in a range between 20 and 100 vol % relative to the nonaqueous solvent, and further at least one compound selected from the group consisting of compounds each having a carbon-nitrogen unsaturated bond, compounds each having a substituent having a carbon-carbon unsaturated bond, and compounds each having a structure of sulfonic acid ester.
Further, Patent Document 15 describes a proposal for improving charge-discharge cycle characteristics, in which a negative electrode includes an element capable of undergoing an alloying reaction with lithium or a compound of an element capable of undergoing an alloying reaction with lithium, and a nonaqueous solvent includes fluorinated ethylene carbonate, and an additive is included such as lithium difluorobis(oxalato)phosphate, lithium difluorooxalatoborate, and lithium tetrafluorooxalatophosphate.
Patent Document 16 discloses an electrolytic solution which can improve a low-temperature property (the ratio of discharge capacities of −20° C./25° C.) at 0° C. or below as well as cycle characteristics and high-temperature storage properties, the electrolytic solution including both a difluoro(bisoxalato)phosphate salt and a tetrafluoro(oxalato)phosphate salt.
It is noted that Patent Document 19 discloses a method of manufacturing a phosphorus-boron complex such as lithium difluorooxalatoborate used as an electrolyte for electrochemical devices. Further, Patent Document 17 discloses a method of manufacturing lithium tris(oxalato)phosphate. Patent Document 18 discloses that cycle characteristics and storage properties can be improved when a diisocyanate compound (hexamethylene diisocyanate), which is one of the compounds having carbon-nitrogen unsaturated bonds, is added to a nonaqueous electrolytic solution.    Nonpatent Document 1 discloses a method of manufacturing a fluoro complex having silicon or the like in the complex center.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. H08-045545    Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2001-006729    Patent Document 3: Japanese Unexamined Patent Application, Publication No. H05-074486    Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2001-057235    Patent Document 5: Japanese Unexamined Patent Application, Publication No. 2007-242411    Patent Document 6: Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2002-519352    Patent Document 7: Japanese Unexamined Patent Application, Publication No. 2006-196250    Patent Document 8: Japanese Unexamined Patent Application, Publication No. 2002-110235    Patent Document 9: Japanese Unexamined Patent Application, Publication No. 2007-335143    Patent Document 10: PCT International Publication No. WO2010/067549    Patent Document 11: PCT International Publication No. WO2012/102259    Patent Document 12: Japanese Unexamined Patent Application, Publication No. 2005-05115 (Japanese Patent No. 4423888)    Patent Document 13: PCT International Publication No. WO2011/142410    Patent Document 14: Japanese Unexamined Patent Application, Publication No. 2013-030284    Patent Document 15: PCT International Publication No. WO2013/132824    Patent Document 16: Japanese Unexamined Patent Application, Publication No. 2011-22193    Patent Document 17: Japanese Unexamined Patent Application (Translation of PCT Application), Publication No. 2003-505464 (Japanese Patent No. 4695802)    Patent Document 18: PCT International Publication No. WO2012/117911    Patent Document 19: Japanese Unexamined Patent Application, Publication No. 2003-137890    Non-Patent Document 1: J. Chem. Soc. (A), 1970, 15, 2569-2574